After doing a major service on brakes like replacing brake lines or rebuilding/replacing a master cylinder, I have always found it difficult to get the brakes bled. The trick I use is to crack the bleeder open and put my finger over the hole while I pump the brake lever. I lift my finger just barely to let some fluid out but maintain some pressure in the system. This has always worked for me except for one case, a bike with a shot master cylinder. Since you bought a used replacement, it has problem been sitting dry without fluid for some time and the seals in the master are shot. Master cylinders are inexpensive and simple to rebuild, I would try that.